Improving the energy output of batteries at sub-zero temperatures is crucial to the long-term application of advanced electronics in extreme environments. This can generally be accomplished by employing high-voltage cathodes, applying Li metal anodes, and improving the electrolyte chemistry to provide facile kinetics at ultralow temperature. However, systems capable of all three of these have seldom been studied. Herein, we demonstrate the design of such a system through solvent fluorination, applying a 1 M LiPF6 in a methyl 3,3,3-trifluoropionate (MTFP)/fluoroethylene carbonate (FEC) (9:1) electrolyte that simultaneously provided high-voltage cathode and Li metal anode reversibility at room temperature. This performance was attributed to the production of fluorine-rich interphases formed in the MTFP-based system, which was investigated by X-ray photoelectron spectroscopy (XPS). Furthermore, the all-fluorinated electrolyte provided 161, 149, and 133 mAh g–1 when discharged at −40, −50, and −60 °C, respectively, far exceeding the performance of the commercial electrolyte. This work provides new design principles for high-voltage batteries capable of ultra-low-temperature operation.

Catastrophic battery failure due to internal short is extremely difficult to detect and mitigate. In order to enable the next‐generation lithium‐metal batteries, a “fail safe” mechanism for internal short is highly desirable. Here, a novel separator design and approach is introduced to mitigate the effects of an internal short circuit by limiting the self‐discharge current to prevent cell temperature rise. A nano‐composite Janus separator—with a fully electronically insulating side contacting the anode and a partially electronically conductive (PEC) coating with tunable conductivity contacting the cathode—is implemented to intercept dendrites, control internal short circuit resistance, and slowly drain cell capacity. Galvanostatic cycling experiments demonstrate Li‐metal batteries with the Janus separator perform normally before shorting, which then results in a gradual increase of internal self‐discharge over >25 cycles due to PEC‐mitigated shorting. This is contrasted by a sudden voltage drop and complete failure seen with a single layer separator. Potentiostatic charging abuse tests of Li‐metal pouch cells result in dendrites completely penetrating the single‐layer separator causing high short circuit current and large cell temperature increase; conversely, negligible current and temperature rise occurs with the Janus separator where post mortem electron microscopy shows the PEC layer successfully intercepts dendrites.

A new solution-based synthesis method to produce a high quality Li2S–P2S5 solid electrolyte was developed by using a strong nucleophile of LiC2H5.

Supercapacitors offer superior cycle life and high power densities, but as energy storage devices, they are limited by self-discharge processes manifested as large potential decay and leakage current, resulting in loss of stored energy and low charging efficiency. To minimize Faradaic side reactions, this Letter has incorporated a sulfonate ion-exchange resin in separators to trap impurities and thereby suppress self-discharge in supercapacitors with PEDOT as redox electrodes. The versatile separator design is generally applicable to organic and aqueous electrolytes and compatible with a pH range of 0− 14, while maintaining the device capacitance and rate performance. Temperature-dependent characteristics were analyzed to identify that the reduction of impurity concentration and diffusion was key to improve potential retention. Compared to devices using commercially available separators, the device here exhibited a lower leakage current and better charging efficiency. It was demonstrated to work with radio frequency energy-harvesting circuits and showed the potential to serve as an energy reservoir for wireless electronic applications.

An in situ formed poly(vinylene carbonate)–lithium iodide gel electrolyte enables stable cycling of lithium metal and a thermal shutdown function.